I entered Throne of Tides via it's portal in the Abyssal Maw to have a quick peak inside. While in the dungeon I was able to mount up so I jumped on my bronze drake and ran around a little (not able to fly). I got to the first trash pull, turned around and exited the dungeon (still mounted). Next thing I see is the Northrend loading screen and then I arrived in Dalaran.

The Throne of the Tides portal inside the instance was set to take you to Dalaran because for a while you couldn't get out of the cavern. In a future beta build, it will take you back to Vashj'ir.